As is known, numerous tachycardial arrhythmias may be reduced instantaneously by the electric stimulation of the heart provided that the stimulation is delivered within a narrow zone of the cardiac cycle called the effective zone.
It has been found that the location of effective zone in the cardiac cycle, as well as the duration per se of the effective zone, varies markedly from one cardiac cycle to another mainly in accordance with the duration of the preceding cardiac cycle called the R--R interval (the interval between consecutive R's); in general, the shorter the duration of the preceding cardiac cycle, the closer the effective zone comes to the beginning of the cycle, with a corresponding reduction in its own length.
Further, according to a method called orthorhythmic stimulation with automatically variable hysteresis, it has been proposed to make the delivery of the electric stimulation pulse dependent upon the duration of the preceding cardiac cycle with a view to ensuring the delivery of the pulse inside the effective zone of the instant cardiac cycle; in practice, and according to the method, the application of the desired electric stimulation of the heart is effected after a period of time called the delay period, which, counting from the natural electrosystole triggering the stimulation, is variable as a function of the duration of the preceding cardiac cycle.
The term "natural electrosystole" is used throughout the present specification as meaning the electrical expression of a natural or induced systole, i.e. the mechanical contraction of the heart.
Yet there may be large and abrupt variations in frequency of the tachycardial arrhythmia treated-namely, when the patient undergoes an intensive drug treatment at the same time. In this case, the effective zone of the cardiac cycle may be sufficiently shortened and/or sufficiently displaced so that, despite the correction provided by the method of orthorhythmic stimulation with automatically variable hysteresis, the stimulation pulse falls outside the effective zone.
Consequently, the stimulation pulse is not able to alleviate cardiac arrhythmias; on the contrary, such a stimulation pulse is capable of provoking an additional contraction of the heart thereby accelerating the heart beat.